[doc] fix ID clash in index.xml

[doc] fix ID clash in index.xml

Without this change I get this error:
```
$ cd doc && make
xsltproc --stringparam library-version `perl -ne 'print "$1\n" if (/:version "(.*)"/)' ../hunchentoot.asd` clixdoc.xsl index.xml > ../www/hunchentoot-doc.html
element a: validity error : ID start already defined

$ xsltproc --version
Using libxml 20902, libxslt 10128 and libexslt 817
xsltproc was compiled against libxml 20902, libxslt 10128 and libexslt 817
libxslt 10128 was compiled against libxml 20902
libexslt 817 was compiled against libxml 20902
```